Machine vision lenses are essential for technical vision systems. To select the correct lens, the vision integrator must know the size, characteristics, and reflectivity of the object being viewed. He must also estimate the working distance and depth of field required to observe thick objects. If the size of the object and images change, the engineer needs to propose a solution that provides system flexibility and does not degrade performance. In harsh operating conditions, the lenses selected must be able to withstand changes in operating conditions.
Role of operating conditions
The most important environmental parameters for vision systems
are reflectivity, lighting, temperature, vibration and contamination. Diaphragms installed in the camera body and hoods (visors above the camera) can reduce glare caused by light scattering. The diaphragm is a light-tight disk with neatly centered holes that allow light to propagate to the matrix in a specific direction. The polarized and diffused radiation of the light source helps to reduce the number of reflective points on the subject.
Lighting, especially monochromatic lighting, can increase the contrast of a subject and improve image quality. Contrast is especially important when using a monochrome camera and can be achieved using additive or difference technologies. With the additive method, the monochromatic light source and the machine vision lens filter match the color of the intended subject of observation. The area around an object reflects or emits light and appears brighter than the object. This technique can be used in applications where a gel or colored liquid is illuminated and examined for individual particles.
Conversely, in a difference system, the filter blocks the light reflected by the area around the subject. As a result, the object appears lighter than its surroundings. An example of the use of filters: control of the tablet making process, where the color of the object is the only distinguishing feature
High ambient temperatures due to thermal expansion of the optical components of the lens can also cause problems. Not all lenses work effectively with changes in temperature, so long working distance lenses are the best solution for monitoring hot objects.
Another important factor is vibration, which can often be mitigated by installing lenses directly on an insulating plate or stand instead of directly in the camera. Heavy lenses often come with mounting clips. If the machine vision lenses
cannot be mounted on a stand or plate, the object on which the lens is mounted can be mounted on an isolation platform. Brackets located on an insulated platform are the most commonly used mounting location for cameras and lenses.
In industrial environments, contaminants can be deposited on the lens surface, so harsh environment optics (HEO), designed specifically for high-quality imaging, use long exposure in harsh environments. Because the optics are sealed, they can be immersed in liquid, resist abrasion and corrosion, dust and mechanical shock.
The area required for an automated vision system and assembly line can vary from a few meters to the size of an entire workshop. Working distance is the distance between the subject and the front edge of the camera lens when focusing the image. This parameter imposes limits on the area required for vision systems and collaboration equipment. In some applications, such as monitoring the state of the lining of a vacuum furnace, the working distance can vary over a wide range, and the operation of the system is ensured by using short-focus objectives or video microscope objectives with a long working distance. In other applications, such as high magnification microscopic observation, the working distance is several centimeters.
The working distance can be varied within certain limits by refocusing the lens. Lenses with infinity focusing distance can focus from the minimum working distance to infinity. Lenses with a finite focusing distance have a defined working range.
The main parameters of the lens that determine the characteristics of the image
The way the system
is mounted, including heavy-duty shielding, must be able to adjust the working distance of the lens. For example, in many cases, the area on a site being viewed by the system may change during the course of a site survey. This may require a vision system or components that can be readjusted to work in different conditions. Many lenses come with stabilized mounts, but if the distance between the subject and the lens is severely limited, the working distance can be altered by adjusting the distance between the lens and the image.
For this, two methods can be used: zooming (scaling) or inserting intermediate rings. Zoom lenses adjust the camera’s field of view without changing the working distance. Some scaling systems are divided into separate units that can be tailored for use in various specific conditions. For metrology and microscopy applications that require magnification down to microns, these objectives can be combined with micro objectives. Zoom lenses are high resolution but can be expensive.
Spacer rings are a more economical solution. They reduce the field of view of the lens by decreasing the working distance. Unfortunately, this leads to distortion and a decrease in resolution, therefore, except for the need to adjust the distance less than 5 mm or special lens design, spacer rings are not recommended for use.
Total Views: 123 ,